This invention relates to stable test strips for the detection of components in liquids.
Recently, test strips have been used to a considerable extent in analysis, especially in clinical chemistry. These are pieces of absorbent material impregnated with the reagents needed for a detection reaction and upon which, shortly after moistening with the fluid to be investigated, the detection reaction takes place. A large number of such rapid tests are known, some of which are commercially available.
An important quality characteristic of rapid test strips is stability, even under extreme storage conditions such as prevail, for example, in the tropics. However, there are a number of highly sensitive rapid test strips which, unfortunately, do not possess this property to a sufficient extent. These include, for example, rapid tests for the detection of blood or hemoglobin and of ketone bodies in the urine.
The rapid test for the detection of blood must contain an organic or inorganic hydroperoxide as oxidation agent, in addition to an oxidation indicator, for example, o-tolidine. The reaction of these two components, which is catalysed by blood in the aqueous phase, cannot be fully prevented on the test strip.
In the case of the rapid test for the detection of ketone bodies, use is made of sodium nitroprusside, together with a strongly alkaline buffer. On the one hand, the detection of ketones only takes place in strongly alkaline solution but, on the other hand, sodium nitroprusside is rapidly decomposed by alkalis.
In order to overcome these difficulties, a large variety of measures have been employed, for example, separate impregnations, micro-encapsulation or dilution with film formers, swelling agents or other separating substances. In some cases, these measures result in a considerable improvement of the stability but, for extreme requirements, they are usually ineffective.
Another means for stabilization is the strict spatial separation of the incompatible reagents. Thus, for example, U.S. Pat. No. 3,511,608 describes rapid tests in which incompatible reagents are impregnated on separate papers. The papers are then fastened together, possible with the insertion of an impermeable separating layer which must be removed prior to use. These rapid tests give satisfactory results when the fluid to be investigated penetrates from above and the upper paper can be removed for the assessment of the reaction color of the lower paper.
However, in the case of commercially available rapid test strips, a form has established itself in which the test paper is wholly immersed in the fluid to be investigated, for example urine, or is even held in a stream of urine and thereafter, without additional manipulation, is compared with comparison colors.
For this purpose, usually small pieces of test paper are applied singly or in groups on to a strip-shaped holder. However, in the case of this type of construction, the measures described in U.S. Pat. No. 3,511,608 do not give a rapid test of sufficient sensitivity. There are two reasons for this: in the first place, both test papers are moistened almost simultaneously since at least the cut edges of the relatively small areas are not covered. The reagents must, therefore diffuse to one another, which takes some time, this time being especially long when one of the reagents is sparingly soluble. On the other hand, the main amount of the reaction color develops between the two papers and is thus not readily visible. For the improvement of the visibility, the upper paper can certainly be of very thin quality but, in this case, the reagent take-up capacity is very limited and considerable difficulties arise in working up due to the low mechanical stability of the thin paper.